GPT: fix error in partitions string doc

The existing partitions-list parsing in cmd/gpt.c passes a value
from gpt_default() to set_gpt_info() that README.gpt suggests
should begin with 'partitions='.  Partition-list strings should
in fact begin with 'uuid_disk', as otherwise the call from
set_gpt_info() to extract_val() to find 'uuid_disk' will fail.
Change README.gpt accordingly.

Changes since v6: none.

Signed-off-by: Alison Chaiken <alison@peloton-tech.com>

doc/README.gpt

@@ -156,10 +156,10 @@ Creating GPT partitions in U-Boot:
 To restore GUID partition table one needs to:
 1. Define partition layout in the environment.
    Format of partitions layout:
-     "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
+     "uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
 	name=kernel,size=60MiB,uuid=...;"
      or
-     "partitions=uuid_disk=${uuid_gpt_disk};name=${uboot_name},
+     "uuid_disk=${uuid_gpt_disk};name=${uboot_name},
 	size=${uboot_size},uuid=${uboot_uuid};"
 
    The fields 'name' and 'size' are mandatory for every partition.
@@ -229,7 +229,7 @@ PARTITION_BASIC_DATA_GUID (EBD0A0A2-B9E5-4433-87C0-68B6B72699C7).
 If you define 'CONFIG_PARTITION_TYPE_GUID', a optionnal parameter 'type'
 can specify a other partition type guid:
 
-     "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
+     "uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
 	name=kernel,size=60MiB,uuid=...,
 	type=0FC63DAF-8483-4772-8E79-3D69D8477DE4;"
 
@@ -251,7 +251,7 @@ Some strings can be also used at the place of known GUID :
 	"lvm"    = PARTITION_LINUX_LVM_GUID
 	           (E6D6D379-F507-44C2-A23C-238F2A3DF928)
 
-    "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
+    "uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
 	name=kernel,size=60MiB,uuid=...,type=linux;"
 
 They are also used to display the type of partition in "part list" command.