SPDX: fix IBM-pibs license identifier

The SPDX License List version 1.19 now contains an official entry for
the IBM-pibs license.  However, instead of our suggestion "ibm-pibs",
the SPDX License List uses "IBM-pibs", with the following rationale:
"The reason being that all other SPDX License List short identifiers
tend towards using capital letters unless spelling a word.  I'd prefer
to be consistent to this end".

Change the license IDs to use the official name.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/Licenses/README b/Licenses/README
index 4196125..9f61192 100644
--- a/Licenses/README
+++ b/Licenses/README
@@ -52,5 +52,5 @@
 eCos license version 2.0			eCos-2.0			eCos-2.0.txt		http://www.gnu.org/licenses/ecos-license.html
 BSD 2-Clause License				BSD-2-Clause	Y		bsd-2-clause.txt	http://spdx.org/licenses/BSD-2-Clause
 BSD 3-clause "New" or "Revised" License		BSD-3-Clause	Y		bsd-3-clause.txt	http://spdx.org/licenses/BSD-3-Clause#licenseText
-IBM PIBS (PowerPC Initialization and		ibm-pibs			ibm-pibs.txt
+IBM PIBS (PowerPC Initialization and		IBM-pibs			ibm-pibs.txt
 	Boot Software) license