sigfox: fix commit hash (pycom#125)

Previous commit 84b9fd1 has only shortened the hash, but failed to update it. This commit fixes it

Author: peter-pycom

esp32/sigfox/README

@@ -6,7 +6,7 @@ There are three prebuilt libraries for SIPY, LOPY4, and FIPY.
 
 git clone -b sigfox git@github.com:pycom/pyupython.git src
 cd src
-git checkout ba89a617
+git checkout 9a5e7a9b
 cd ../..
 make BOARD=LOPY4 TARGET=sigfox
 make BOARD=FIPY TARGET=sigfox