[devel] Fix spelling of "conversion" in manual.

libpng-manual.txt

@@ -1451,8 +1451,8 @@ the information.  If, instead, you want to convert the image to an opaque
 version with no alpha channel use png_set_background; see below.
 
 As of libpng version 1.5.2, almost all useful expansions are supported, the
-major ommissions are convertion of grayscale to indexed images (which can be
-done trivially in the application) and convertion of indexed to grayscale (which
+major ommissions are conversion of grayscale to indexed images (which can be
+done trivially in the application) and conversion of indexed to grayscale (which
 can be done by a trivial manipulation of the palette.)
 
 In the following table, the 01 means grayscale with depth<8, 31 means

libpng.3

@@ -2402,8 +2402,8 @@ the information.  If, instead, you want to convert the image to an opaque
 version with no alpha channel use png_set_background; see below.
 
 As of libpng version 1.5.2, almost all useful expansions are supported, the
-major ommissions are convertion of grayscale to indexed images (which can be
-done trivially in the application) and convertion of indexed to grayscale (which
+major ommissions are conversion of grayscale to indexed images (which can be
+done trivially in the application) and conversion of indexed to grayscale (which
 can be done by a trivial manipulation of the palette.)
 
 In the following table, the 01 means grayscale with depth<8, 31 means