Update source/faq/concurrency.txt

Simple typo - word missing in the JS operations paragraph

Author: comerford

source/faq/concurrency.txt

@@ -233,9 +233,9 @@ secondaries, in a two phases:
 What kind of concurrency does MongoDB provide for JavaScript operations?
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-A single :program:`mongod` can only *single* JavaScript operation at
-once. Therefore, operations that rely on JavaScript cannot run
-concurrently; however, the :program:`mongod` can often run other
+A single :program:`mongod` can only run a *single* JavaScript
+operation at once. Therefore, operations that rely on JavaScript cannot
+run concurrently; however, the :program:`mongod` can often run other
 database operations concurrently with the JavaScript execution. This
 limitation with JavaScript affects the following operations: