Fix typos in user documentation

gpdb-doc/dita/admin_guide/query/topics/json-data.xml

@@ -870,7 +870,7 @@ Array with n elements > array with n - 1 elements</pre>
             <thead>
               <row>
                 <entry>Function</entry>
-                <entry>Agument Types</entry>
+                <entry>Argument Types</entry>
                 <entry>Return Type</entry>
                 <entry>Description</entry>
               </row>

gpdb-doc/dita/ref_guide/function-summary.xml

@@ -898,7 +898,7 @@ SELECT foo();
             <thead>
               <row>
                 <entry>Function</entry>
-                <entry>Agument Types</entry>
+                <entry>Argument Types</entry>
                 <entry>Return Type</entry>
                 <entry>Description</entry>
               </row>

gpdb-doc/dita/ref_guide/sql_commands/ALTER_FOREIGN_TABLE.xml

@@ -51,7 +51,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <varname>name</varname>
                 <plentry>
                     <pt>IF EXISTS</pt>
                     <pd>
-                        <p>If you specify <codeph>IF EXISTS</codeph> and the foriegn table does not exist, no error is thrown. Greenplum Database issues a notice instead.</p>
+                        <p>If you specify <codeph>IF EXISTS</codeph> and the foreign table does not exist, no error is thrown. Greenplum Database issues a notice instead.</p>
                     </pd>
                 </plentry>
                 <plentry>