README: Fix some typos (pull request #109)

Author: endolith

README.md

@@ -176,7 +176,7 @@ extensions](http://johnmacfarlane.net/pandoc/README.html#tables):
     eggs      451
     bacon       0
 
-`github` follows the conventions of Github flavored Markdown. It
+`github` follows the conventions of GitHub flavored Markdown. It
 corresponds to the `pipe` format without alignment colons:
 
     >>> print(tabulate(table, headers, tablefmt="github"))
@@ -668,9 +668,9 @@ as a number imply that `tabulate`:
 It may not be suitable for serializing really big tables (but who's
 going to do that, anyway?) or printing tables in performance sensitive
 applications. `tabulate` is about two orders of magnitude slower than
-simply joining lists of values with a tab, coma or other separator.
+simply joining lists of values with a tab, comma, or other separator.
 
-In the same time `tabulate` is comparable to other table
+At the same time, `tabulate` is comparable to other table
 pretty-printers. Given a 10x10 table (a list of lists) of mixed text and
 numeric data, `tabulate` appears to be slower than `asciitable`, and
 faster than `PrettyTable` and `texttable` The following mini-benchmark
@@ -714,7 +714,7 @@ On Linux `tox` expects to find executables like `python2.6`,
 `C:\Python26\python.exe`, `C:\Python27\python.exe` and
 `C:\Python34\python.exe` respectively.
 
-To test only some Python environements, use `-e` option. For example, to
+To test only some Python environments, use `-e` option. For example, to
 test only against Python 2.7 and Python 3.6, run:
 
     tox -e py27,py36