Update README.md

Changed 'period' to 'frequency' in options.
diff --git a/README.md b/README.md
index 1b6c9bf..7dc7044 100644
--- a/README.md
+++ b/README.md
@@ -120,7 +120,7 @@
                                                            accelerometerOptions);
 
 - __accelerometerOptions__: An object with the following optional keys:
-  - __period__: requested period of calls to accelerometerSuccess with acceleration data in Milliseconds. _(Number)_ (Default: 10000)
+  - __frequency__: requested frequency of calls to accelerometerSuccess with acceleration data in Milliseconds. _(Number)_ (Default: 10000)
 
 
 ###  Example