Fixed typos and grammar in lib/Syntax/README.md

Added a semicolon to a run-on sentence, and restored the original commas there.

Author: NevinBR

lib/Syntax/README.md

@@ -163,8 +163,8 @@ struct YourStruct {}
 At any point in the building process, you can call `build()` and get a
 reasonably formed Syntax node (i.e. with no raw `nullptr`s) using what you've
 provided to the builder so far. Anything that you haven't supplied is marked as
-*missing*. This is essentially what the parser does, so—looking forward to
-future adoption—the builders are designed with the parser in mind, with the
+*missing*. This is essentially what the parser does; so, looking forward to
+future adoption, the builders are designed with the parser in mind, with the
 hope that we can better specify recovery behavior and incremental (re-)parsing.
 
 **Example**