Wrong function name in pgstatfuncs.c

Since pgstatfuncs.c was refactored, the comments for synthesized
function names are significant to find the function body.

I happened to find a misspelling among the function name
comments. "pg_stat_get_mods_since_analyze" should be
"pg_stat_get_mod_since_analyze".

Upon checking the file using a rudimentary script, I found no other
similar mistakes in the same file.

(FWIW, I also feel that these macros might be going a bit too far by
synthesizing even the function names.)

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center