fix version

Author: reneeli@microsoft.com

src/aks-preview/HISTORY.rst

@@ -12,7 +12,7 @@ To release a new version, please select a new version number (usually plus 1 to
 Pending
 +++++++
 
-15.0.0b1`
+15.0.0b1
 ++++++++
 * [BREAKING CHANGE] Change `--vm-sizes` for VirtualMachines manual profile to awalys support single SKU size.