[metapost] some typos in the MetaPost User Manual

[胡亚捷 (Hu Yajie)]

Hi, I skimmed through mpman.pdf and found some typos:
1. The last entry of Table 12 (page 91) should say "(x<suffix>, y<suffix>)"
instead of "x<suffix>, y<suffix>)".
2. Abbreviations like "ca." are erroneously followed by intersentence spaces.
3. "PostScript"/"Postscript" is inconsistently capitalized.
4. In the language syntaxes (e.g. page 92), would it be better to put
thinspaces around the terminal symbols, to show that concatenation is on the
token level and not the character level?

(I'm not a MetaPost user, so I didn't read mpman as carefully as mfbook.)