glossary: Use proper U-Boot spelling

Fix inconsistent capitalization; "U-Boot" is the correct spelling.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

Author: lbmeng

source/glossary.rst

@@ -23,7 +23,7 @@ Firmware Handoff documentation.
       Application Processor
 
    Blob List
-      Bloblist is an U-boot implementation of the Firmware Handoff protocol
+      Bloblist is an U-Boot implementation of the Firmware Handoff protocol
 
    DT
       Device Tree