Fix README to install spanner instead of datastore. (#7301)

Author: daniellehanks

spanner/README.rst

@@ -71,7 +71,7 @@ Mac/Linux
     pip install virtualenv
     virtualenv <your-env>
     source <your-env>/bin/activate
-    <your-env>/bin/pip install google-cloud-datastore
+    <your-env>/bin/pip install google-cloud-spanner
 
 
 Windows
@@ -82,7 +82,7 @@ Windows
     pip install virtualenv
     virtualenv <your-env>
     <your-env>\Scripts\activate
-    <your-env>\Scripts\pip.exe install google-cloud-datastore
+    <your-env>\Scripts\pip.exe install google-cloud-spanner
 
 
 Example Usage