btrfs: fix string and comment grammatical issues and typos

Signed-off-by: Nicholas D Steeves <nsteeves@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
index 318b048..e0715fc 100644
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c
@@ -62,7 +62,7 @@
 
 /**
  * free_extent_map - drop reference count of an extent_map
- * @em:		extent map being releasead
+ * @em:		extent map being released
  *
  * Drops the reference out on @em by one and free the structure
  * if the reference count hits zero.