docs/LoongArch: Add I14 description

I14 is also a kind of immediate operand in instruction, like I8/I12/I16/
I21/I26, add it in the English documentation.
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Documentation/loongarch/introduction.rst

@@ -221,7 +221,7 @@ I26         Opcode + I26L + I26H
 =========== ==========================
 
 Rd is the destination register operand, while Rj, Rk and Ra ("a" stands for
-"additional") are the source register operands. I8/I12/I16/I21/I26 are
+"additional") are the source register operands. I8/I12/I14/I16/I21/I26 are
 immediate operands of respective width. The longer I21 and I26 are stored
 in separate higher and lower parts in the instruction word, denoted by the "L"
 and "H" suffixes.