fix variable invalid when key only one character (#2377)

api/core/prompt/prompt_template.py

@@ -1,6 +1,6 @@
 import re
 
-REGEX = re.compile(r"\{\{([a-zA-Z_][a-zA-Z0-9_]{1,29}|#histories#|#query#|#context#)\}\}")
+REGEX = re.compile(r"\{\{([a-zA-Z_][a-zA-Z0-9_]{0,29}|#histories#|#query#|#context#)\}\}")
 
 
 class PromptTemplateParser: