Update xAPI.md

Edited lines 214-217 for clarity.
Deleted "by nature" from line 223.
Line 225. Changed that to whom (because it's about people). 
Line 228. Inserted the before xAPI.
Line 242. Deleted stray quote mark at end.
Line 278. Deleted extra space in Immutable definition.
Line 302. Edited punctuation of the however clause.
Line 307. Changed regards to regard.
Line 333. Fixed typo in "the" in Tin Can definition.
Line 345. Changed comma to semicolon.
Line 444. Changed period to semicolon.
Line 447. Inserted parentheses.
Line 475. Changed period to semicolon.
Line 476. Added period at end.
Line 484. Should description include word "Required"? (per line 474)
Line 491. Changed period to semicolon.
Line 494. Deleted extra space.
Line 526. Changed emails to email addresses. 
Line 548. Corrected anonimity to anonymity.