Issue #25523: Correct "a" article to "an" article

This changes the main documentation, doc strings, source code comments, and a
couple error messages in the test suite. In some cases the word was removed
to fix the grammar.
diff --git a/PC/os2emx/Makefile b/PC/os2emx/Makefile
index bf2e486..613da64 100644
--- a/PC/os2emx/Makefile
+++ b/PC/os2emx/Makefile
@@ -102,7 +102,7 @@
   CFLAGS+=	-DPREFIX=$(DQUOTE)$(LIB_DIR)$(DQUOTE)
 endif
 
-# We're using the OMF format since EMX's ld has a obscure bug
+# We're using the OMF format since EMX's ld has an obscure bug
 # because of which it sometimes fails to build relocations
 # in .data segment that point to another .data locations
 # (except for the final linking if the .EXEs)