Update fielddata.asciidoc

Spelling correction

docs/reference/index-modules/fielddata.asciidoc

@@ -29,7 +29,7 @@ example, can be set to `5m` for a 5 minute expiry.
 === Field data circuit breaker
 The field data circuit breaker allows Elasticsearch to estimate the amount of
 memory a field will required to be loaded into memory. It can then prevent the
-field data loading by raising and exception. By default the limit is configured
+field data loading by raising an exception. By default the limit is configured
 to 60% of the maximum JVM heap. It can be configured with the following
 parameters: