bigquerystorage: address doc nits

-updates description in pom.xml for the api
-updates the readme to reflect this is actually shipped with -alpha
qualifier rather than beta.

Caveat:  I'm not familiar with how versions get revved for releases, so
this may need addressed elsewhere or it may break for the next release.

It looks like the readme and pom diverged for bigquerystorage
at googleapis#3978

google-cloud-clients/google-cloud-bigquerystorage/README.md

@@ -20,16 +20,16 @@ Add this to your pom.xml file
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-bigquerystorage</artifactId>
-  <version>0.81.0-beta</version>
+  <version>0.81.0-alpha</version>
 </dependency>
 ```
 If you are using Gradle, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-bigquerystorage:0.81.0-beta'
+compile 'com.google.cloud:google-cloud-bigquerystorage:0.81.0-alpha'
 ```
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-bigquerystorage" % "0.81.0-beta"
+libraryDependencies += "com.google.cloud" % "google-cloud-bigquerystorage" % "0.81.0-alpha"
 ```
 [//]: # ({x-version-update-end})
 

google-cloud-clients/google-cloud-bigquerystorage/pom.xml

@@ -7,7 +7,7 @@
     <name>Google Cloud Bigquery Storage</name>
     <url>https://github.com/googleapis/google-cloud-java/tree/master/google-cloud-clients/google-cloud-bigquerystorage</url>
     <description>
-        bigquery storage fill in later
+       BigQuery Storage API provides fast access to BigQuery table data. 
     </description>
     <parent>
         <groupId>com.google.cloud</groupId>