fix(cpus): fix a typo in errata doc

Commit@af5ae9a73f67dc8c9ed493846d031b052b0f22a0
Adding a Cortex-A720-AE erratum 3699562 has a typo in CPU name
for the errata, it is for Cortex-A720-AE but had incorrectly
mentioned as Cortex-A715_AE.

Change-Id: I2332a3fcaf56a7aaab5a04e3d40428cc746d2d46
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index a871c59..67f29f0 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -1030,7 +1030,7 @@
 For Cortex-A720_AE, the following errata build flags are defined :
 
 -  ``ERRATA_A720_AE_3699562``: This applies errata 3699562 workaround
-   to Cortex-A715_AE CPU. This needs to be enabled for revisions r0p0.
+   to Cortex-A720_AE CPU. This needs to be enabled for revisions r0p0.
    It is still open.
 
 For Cortex-A725, the following errata build flags are defined :