fix several typos

src/backend/access/bitmap/README

@@ -14,7 +14,7 @@ indexes are much less expensive to construct than b-trees.
 Hybrid Run-Length (HRL) equality encoding bitmap index
 ------------------------------------------------------
 
-HRL is the bitmap encoding mechanism used in this implemention. In HRL,
+HRL is the bitmap encoding mechanism used in this implementation. In HRL,
 each vector is represented in two sections: the header section and the
 content section. The header section contains bits, each of which
 corresponds to a word in the content section. If a bit in the header

src/backend/access/nbtree/README

@@ -46,7 +46,7 @@ Lehman and Yao don't require read locks, but assume that in-memory
 copies of tree pages are unshared.  Postgres shares in-memory buffers
 among backends.  As a result, we do page-level read locking on btree
 pages in order to guarantee that no record is modified while we are
-examining it.  This reduces concurrency but guaranteees correct
+examining it.  This reduces concurrency but guarantees correct
 behavior.  An advantage is that when trading in a read lock for a
 write lock, we need not re-read the page after getting the write lock.
 Since we're also holding a pin on the shared buffer containing the

src/backend/cdb/cdbvars.c

@@ -533,7 +533,7 @@ assign_gp_session_role(const char *newval, bool doit, GucSource source __attribu
 
 
 /*
- * Assign hook routine for "gp_role" option.  This variablle has context
+ * Assign hook routine for "gp_role" option.  This variable has context
  * PGC_SUSET so that is can only be set by a superuser via the SET command.
  * (It can also be set using an option on postmaster start, but this isn't
  * interesting beccause the derived global CdbRole is always set (along with
@@ -615,7 +615,7 @@ assign_gp_role(const char *newval, bool doit, GucSource source)
 
 
 /*
- * Assign hook routine for "gp_connections_per_thread" option.  This variablle has context
+ * Assign hook routine for "gp_connections_per_thread" option.  This variable has context
  * PGC_SUSET so that is can only be set by a superuser via the SET command.
  * (It can also be set in config file, but not inside of PGOPTIONS.)
  *