chore(AIP-121): fix some grammar and punctuation issues (#1470)

Author: weixifan

aip/general/0121.md

@@ -101,7 +101,7 @@ operations (either successful or with an error, long-running operation, or
 synchronous) **must** mean that the state of the resource's existence and all
 user-settable values have reached a steady-state.
 
-[output only][] values unrelated to the resource [state][] **should** also have
+[Output only][] values unrelated to the resource [state][] **should** also have
 reached a steady-state for values that are related to the resource [state][].
 
 Examples include:
@@ -121,8 +121,8 @@ ensuring that resources immediately reflect steady user state after an operation
 is complete ensures clients can rely on method completion as a signal to begin
 the next operation.
 
-[output only][] fields ideally would follow the same guidelines, but as
-these fields can often represent a resources live state, it's sometimes
+[Output only][] fields ideally would follow the same guidelines, but as
+these fields can often represent a resource's live state, it's sometimes
 necessary for these values to change after a successful mutation operation to
 reflect a state change.