[main][Docs] Fix spelling errors across documentation (#6649)

Fix various spelling mistakes in the project documentation to improve
clarity and correctness.
- vLLM version: v0.15.0
- vLLM main:
d7e17aaacd

---------

Signed-off-by: SlightwindSec <slightwindsec@gmail.com>

docs/source/user_guide/feature_guide/Multi_Token_Prediction.md

@@ -109,7 +109,7 @@ if self.speculative_config:
         got {self.decode_threshold}"
 ```
 
-## Limitation
+## Limitations
 
 - Due to the fact that only a single layer of weights is exposed in DeepSeek's MTP, the accuracy and performance are not effectively guaranteed in scenarios where MTP > 1 (especially MTP ≥ 3). Moreover, due to current operator limitations, MTP supports a maximum of 15.
 - In the fullgraph mode with MTP > 1, the capture size of each aclgraph must be an integer multiple of (num_speculative_tokens + 1).