Update pre-release label (#4889)

Author: mmitche

eng/Versions.props

@@ -3,7 +3,7 @@
     <MajorVersion>10</MajorVersion>
     <MinorVersion>0</MinorVersion>
     <PatchVersion>0</PatchVersion>
-    <PreReleaseVersionLabel>alpha</PreReleaseVersionLabel>
+    <PreReleaseVersionLabel>preview</PreReleaseVersionLabel>
     <PreReleaseVersionIteration>1</PreReleaseVersionIteration>
     <!-- Enable to remove prerelease label. -->
     <StabilizePackageVersion Condition="'$(StabilizePackageVersion)' == ''">false</StabilizePackageVersion>